1. Field of the Invention
The present invention relates to a chucking device of a workpiece to be ground in a grinding machine which requires an extremely high precision of grinding.
The present invention can be used, for example, to chuck a ferrule which is used as an optical connector. The ferrule can be used to optically connect opposed optical fibers which are inserted in an axial bore of the ferrule and which butt against each other. Generally, the axial bore of the ferrule is approximately in the order of 0.1 mm of diameter, so that the optical fibers can be closely fitted in the axial bore. The ferrule can be made of a metal sleeve or the like.
Such a small ferrule must be ground and finished with a high precision, so that the outer periphery of the ferrule is coaxial to the small axial bore thereof. Usually, the tolerance of the outer diameter of the ferrule per se is on the order of approximately 1 .mu.m or less, the coaxial coincidence between the outer diameter and the axial bore of the ferrule requires the same extent of precision. The ferrule, a small workpiece, usually is about 2 to 4 mm in outer diameter and about 10 mm in length. Accordingly, it is necessary to use a special chucking device which can hold a very narrow area or short length of the ferrule to chuck the same.
In addition, in order to automate the grinding operation, including the chucking operation, it is necessary to release the chuck when the grinding operation is at rest and to self-clamp the ferrule in the course of the grinding operation.
2. Description of the Related Art
In a known chucking device, for example, disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-107809 which is considered closest to the present invention among those the applicant is aware of, the workpiece to be ground, such as a ferrule, can be held by a pair of clamp plates which have elongated openings with circular arcs and which can be moved away from and close to each other to enlarge or reduce the substantially eye-shaped or oval opening defined by the circular arcs of the opposed elongated openings of the clamp plates in order to release and hold the ferrule inserted in the circular opening. Since the ferrule is chucked, however, substantially at diametrically opposed two points (exactly speaking, two lines) of the inner periphery of the eye-shaped opening by a frictional force, sufficient chucking of the ferrule cannot be achieved, resulting in an instable clamping of the ferrule. In addition to the instability, if the axial bore of the ferrule is not coaxial to the outer periphery of the ferrule, that is, if the ferrule has an eccentric axial bore, the ferrule which is held between and by sharp ends of opposed centers of a grinder is eccentric to or deviated from the center axis connecting the opposed centers.
The eccentricity results in a bending force acting on the ferrule or an unbalanced clamp of the ferrule, resulting in a bad influence on the precision of grinding.
The primary object of the present invention is, therefore, to eliminate the aforementioned drawbacks of the prior art.